Which render engine should I use for export to Unity
Rendering or image synthesis is the automatic process of generating a photorealistic or non-photorealistic image from a 2D or 3D model


(see Wikipedia)

This implies generating an Image, which will be a projection of your model to a 2-dimensional space. Anyway, as long as you are not programming a sprite-based game, I don't think what you want to have is an image of your model, but a model to be rendered within the game.

For that purpose rendering in Blender is completely irrelevant and hence is the rendering engine. 

Sidenote: If you are asking which rendering engine to use with Blender for photorealistic modeling in general, please drop the Unity from your question.